Laser technology was first introduced in 1960 and is a kind of light that is enhanced by the stimulation of radiation. Lasers are widely used because of their good monochromaticity, strong directionality, and high brightness. The principle of laser technology is: when the energy of light or current strikes a certain excited substance such as a crystal or an atom, the electron of the atom reaches the excited high energy state, when these electrons return to a calm low energy state. The atom will emit photons to release excess energy; then, the emitted photons will hit other atoms, trigger more atoms to produce photons, trigger a series of "chain reactions", and all move toward the same side. , forming a strong and concentrated light in a certain direction. This kind of light is called a laser. The laser is almost a monochromatic light wave with a very narrow frequency range and high energy concentration in a narrow direction. Therefore, various materials can be punched by the focused laser beam. Lasers have a wide range of applications because of this property.
A semiconductor laser is a coherent radiation source. To enable it to produce a laser, three basic conditions must be met:
1. Gain condition: establish the reverse distribution of carriers in the lasing medium (active region). The electron energy in the semiconductor is composed of a series of energy bands close to continuous energy levels, so in the semiconductor In order to achieve the population inversion, it is necessary to have a large number of electrons at the bottom of the low-energy valence band between the two energy band regions, which is much larger than the number of holes at the top of the low-energy valence band. The heterojunction is forward biased and the necessary carriers are injected into the active layer. The electrons are excited from a lower energy valence band to a higher energy conduction band. When a large number of electrons in a state in which the number of particles is inverted are combined with holes, a stimulated emission is generated.
2. To obtain the coherent stimulated radiation, the stimulated radiation must be feedbacked multiple times in the optical cavity to form a laser oscillation. The cavity of the laser is formed by the natural cleavage plane of the semiconductor crystal as a mirror, usually The end that does not emit light is plated with a high-reverse multilayer dielectric film, and the light-emitting surface is plated with an anti-reflection film. For the F-p cavity (Fabry-Perot cavity) semiconductor laser, it is convenient to use the natural cleavage plane perpendicular to the plane of the P-n junction to form the F-P cavity.
3. In order to form a stable oscillation, the laser medium must be able to provide a sufficiently large gain to compensate for the optical loss caused by the cavity and the loss caused by the laser output from the cavity surface, and continuously increase the light field in the cavity. This requires a sufficiently strong current injection, that is, there is sufficient population inversion, and the higher the degree of particle inversion, the greater the gain obtained, that is, the requirement that a certain current threshold condition must be met. When the laser reaches the threshold, light having a specific wavelength can resonate in the cavity and be amplified, and finally a laser is formed to continuously output.
It can be seen that in semiconductor lasers, the dipole transition of electrons and holes is a basic light emission and light amplification process. For new semiconductor lasers, quantum wells are currently recognized as the fundamental driving force for the development of semiconductor lasers. The issue of whether quantum wires and quantum dots can make full use of quantum effects has been extended to this century. Scientists have tried to make quantum dots in various materials using self-organizing structures, and GaInN quantum dots have been used in semiconductor lasers. In addition, scientists have also made another type of quantum cascade laser with stimulated radiation based on a transition from a sub-level of the semiconductor conduction band to a lower level of the same band, since only The electrons in the conduction band participate in this process, so it is a unipolar device.
Laser spectroscopy is a spectroscopy technique using laser as a light source. It is mainly used for molecular spectroscopy, plasma physics, scientific applications of high-order harmonic generation, air pollution monitoring, and cancer diagnosis. The use of semiconductor lasers as a source of laser spectroscopy has many advantages. It has small volume, low input energy, long life, good coordination and low price.
Semiconductor lasers have the advantages of small size, low cost, long life, wavelength selectivity, and stable output power. They are especially suitable for medical devices, and their clinical applications cover almost all other types of laser applications. For example, a low-power 810nm near-infrared semiconductor laser has a strong laser penetration capability at this wavelength, and the refractive interstitial has the least absorption and a large adjustable spot diameter. It is the most commonly used heat source in ophthalmology and can be used for the treatment of glaucoma and silicone oil injection. Postoperative refractory high intraocular pressure and photocoagulation and fixation of the retina; 810nm semiconductor laser can be well absorbed by melanin in the hair follicle, producing thermal effects, destroying hair follicles, and completing hair removal; high-power semiconductor lasers are also widely used in tumors. Laser cutting, coagulation surgery. These all provide further protection for human health.
Semiconductor lasers are also widely used in information acquisition, transmission, storage and processing, and display. In the 21st century, with the development of optical fiber communication, semiconductor laser light as a light source in optical fiber communication system is a key component and is the core part of the whole system. Short-distance optical fiber communication uses single-mode optical fiber and semiconductor laser with wavelength of 130-150 nm. Array semiconductor lasers for space communication. The global fiber-optic communication market has broad prospects, so the market prospects for semiconductor lasers are also very good.